Researchers at the University of California, San Francisco (UCSF) have identified the lynchpin that activates brown fat cells, which burn fat molecules instead of storing them, making them the focus of pharmaceutical research aimed at fighting obesity. The importance of brown fat in burning calories to generate heat in humans—as distinct from other mammals such as mice and bears—has only been appreciated in recent years. In humans, brown fat was until recently thought to be present only in infants. Scientists now know that adult humans possess small but significant deposits of brown fat—to go along with the all-too-obvious white fat that especially plagues the growing ranks of the obese.